Start moving embedding API towards "slot" register-API.

- wrenGetArgumentCount() -> wrenGetSlotCount()
- wrenGetArgument___() -> wrenGetSlot___()

Also, the get functions assert that the value is the right type instead
of checking at runtime. This puts the onus on the caller to be safe,
but maximizes performance.

src/optional/wren_opt_meta.c

@@ -18,7 +18,7 @@ void metaCompile(WrenVM* vm)
       : AS_CLOSURE(callingFn)->fn->module;
 
   // Compile it.
-  ObjFn* fn = wrenCompile(vm, module, wrenGetArgumentString(vm, 1), false);
+  ObjFn* fn = wrenCompile(vm, module, wrenGetSlotString(vm, 1), false);
   if (fn == NULL) return;
 
   // Return the result. We can't use the public API for this since we have a

src/optional/wren_opt_random.c

@@ -45,7 +45,7 @@ static void randomAllocate(WrenVM* vm)
 
 static void randomSeed0(WrenVM* vm)
 {
-  Well512* well = (Well512*)wrenGetArgumentForeign(vm, 0);
+  Well512* well = (Well512*)wrenGetSlotForeign(vm, 0);
 
   srand((uint32_t)time(NULL));
   for (int i = 0; i < 16; i++)
@@ -56,9 +56,9 @@ static void randomSeed0(WrenVM* vm)
 
 static void randomSeed1(WrenVM* vm)
 {
-  Well512* well = (Well512*)wrenGetArgumentForeign(vm, 0);
+  Well512* well = (Well512*)wrenGetSlotForeign(vm, 0);
 
-  srand((uint32_t)wrenGetArgumentDouble(vm, 1));
+  srand((uint32_t)wrenGetSlotDouble(vm, 1));
   for (int i = 0; i < 16; i++)
   {
     well->state[i] = rand();
@@ -67,17 +67,17 @@ static void randomSeed1(WrenVM* vm)
 
 static void randomSeed16(WrenVM* vm)
 {
-  Well512* well = (Well512*)wrenGetArgumentForeign(vm, 0);
+  Well512* well = (Well512*)wrenGetSlotForeign(vm, 0);
 
   for (int i = 0; i < 16; i++)
   {
-    well->state[i] = (uint32_t)wrenGetArgumentDouble(vm, i + 1);
+    well->state[i] = (uint32_t)wrenGetSlotDouble(vm, i + 1);
   }
 }
 
 static void randomFloat(WrenVM* vm)
 {
-  Well512* well = (Well512*)wrenGetArgumentForeign(vm, 0);
+  Well512* well = (Well512*)wrenGetSlotForeign(vm, 0);
 
   // A double has 53 bits of precision in its mantissa, and we'd like to take
   // full advantage of that, so we need 53 bits of random source data.
@@ -97,7 +97,7 @@ static void randomFloat(WrenVM* vm)
 
 static void randomInt0(WrenVM* vm)
 {
-  Well512* well = (Well512*)wrenGetArgumentForeign(vm, 0);
+  Well512* well = (Well512*)wrenGetSlotForeign(vm, 0);
 
   wrenReturnDouble(vm, (double)advanceState(well));
 }